Atwood Hydro Flame furnace shoots cold air then shuts off

I also thought that an onboard 12v batt was required for proper operation of 12v systems. Perhaps put a battery on (I assume you have one somewhere) and see if it fixes your problem. Just because some things work on 12v with the converter only....does not mean that the converter can supply enough power to run several things (fridge/furnace/lights etc).